Progressive still frame mode

ABSTRACT

A still image is progressively transmitted over a communications channel by computing, encoding, and outputting for transmission difference information representing the difference between the still image and a previously-transmitted image that has been stored at a remote location. This process is repeated, with additional difference information being generated to supplement the image received at the remote location, such that the quality of the image at the remote location improves over time. The difference information may be encoded in a format compatible with existing video compression standards and processed by a receiving terminal regardless of the presence of still mode capability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of patent application Ser. No.08/768,894, filed Dec. 17, 1996.

FIELD OF THE INVENTION

The present invention relates to image transmission. More particularly,the present invention relates to a method and apparatus forprogressively transmitting a still image.

BACKGROUND OF THE INVENTION

Videocommunication equipment, such as videoconferencing systems andvideophone devices, have enabled people to communicate visually withouthaving to travel to a common location. As a result, communicationparticipants can be separated by large distances.

A typical videoconferencing application uses a video camera to capture aseries of images of a target, such as a meeting participant or adocument. The series of images is encoded as a data strewn andtransmitted over a communications channel to a remote location. Forexample, the data stream may be transmitted over a phone line, anintegrated services digital network (ISDN) line, or the Internet. Theencoding process is typically implemented using a digital videocoder/decoder (codec), which divides the images into blocks andcompresses the blocks according to a video compression standard, such asthe ITU-T H.263 and H.261 recommendations. In standards of this type, ablock may be compressed independent of the previous image or as adifference between the block and part of the previous image.

In a typical videoconferencing system, the data stream is received at aremote location, where it is decoded into a series of images, which maybe viewed at the remote location. Depending on the equipment used, thisprocess typically occurs at a rate of one to thirty frames per second.

In some videoconferencing applications, it is desirable to transmit ahigh quality still image. Until the image is completely received anddecoded, the receiving terminal is often unaware of its content. Somedecoders decode and display a block only after they have received thecomplete image. With the image being transmitted as a series of blocks,considerable delay is often involved in transmitting the entire image.For example, in applications where the available bandwidth fortransmitting data is small, transmission of a 352×288 pixel image mayrequire up to a minute. In order to transmit still images more quickly,a lower image quality may be used for compressing the image.

SUMMARY OF THE INVENTION

Generally, the present invention provides methods and arrangements forprogressively transmitting and receiving high quality still images. Inone particular embodiment of the invention, a location has stored apreviously-transmitted image. Difference information is computed thatrepresents a difference between the previously-transmitted image and thestill image. The difference information is encoded as encoded data in aformat capable of being processed at the second location. The encodeddata is output for transmission over the communications channel. Arefined image is reconstructed using the previously-transmitted imageand the difference information, and is stored. The quality of therefined image is increased until it satisfies a quality threshold byiteratively repeating the steps of computing the difference information,encoding the difference information, outputting the encoded data,constructing the refined image, and storing the refined image. Duringeach iteration, a parameter is adjusted to increase the quality of therefined image. In another embodiment of the present invention, a systemprovides these capabilities.

In another method embodiment of the present invention, a still image istransmitted from a first location to a second location using acommunications channel. The second location has stored a first imagethat was received in response to a previous still image request.Difference information representing a difference between the first imageand the still image is computed. This difference information is encodedas encoded data in a format capable of being processed at the secondlocation. The encoded data is output for transmission over thecommunications channel. A refined image is constructed using the firstimage and the difference information, and is stored. A quality of therefined image is increased until it satisfies a quality threshold byiteratively repeating these steps, adjusting a parameter during eachiteration to increase the quality of the refined image.

In another embodiment of the invention, a system for transmitting astill image at a desired quality using a communications channel isprovided. The system includes a still mode control block that isconfigured and arranged to selectively enable a still mode. A firstmemory is configured and arranged to, when the still mode is notenabled, store a series of images in succession. When the still mode isenabled, the first memory stores one of the series of images as thestill image to be transmitted. A difference information generator isconfigured and arranged to generate difference information representinga difference between a reconstructed image and the still image. Avariable quality encoder is configured and arranged to encode thedifference information. The difference information is encoded as encodeddata representing the difference information at a variable quality thatis successively increased until a quality threshold is satisfied. Animage reconstructor is configured and arranged to construct thereconstructed image as a function of the difference information.Initially, the reconstructed image comprises a previously-transmittedimage. A second memory is configured and arranged to store thereconstructed image.

In still another embodiment of the invention, a method for generating animage is provided. A image is constructed based on data received from acommunications channel. The image represents a previously-transmittedimage and is stored in a memory. Difference information is received fromthe communications channel, and a new image is constructed based on theimage and on the difference information and is stored in the memory.This process is repeated, with the quality of the new image beingincreased at each iteration until the quality satisfies a thresholdcondition. This method can be performed by a system according to anotherembodiment of the invention.

In another method embodiment of the invention, a still image isgenerated at a receiving terminal based on a first image received from acommunications channel in response to a previous still image request bystoring the first image in a memory. Difference information is receivedfrom the communications channel and is used with the image stored in thememory in constructing a new image. The new image is stored in thememory. These steps are repeated until a quality of the new imagesatisfies a threshold condition by increasing the quality of the newimage at each iteration at least in part by incorporating the differenceinformation into the new image.

In yet another embodiment of the invention, a system for generating astill image having a desired quality is provided. The system includes astill mode enabler, which is configured and arranged to selectivelyenable a still mode, and a decoder, which configured and arranged todecode data received from a communications channel. An imageconstructor, responsive to the decoder, includes a summing block foriteratively incorporating difference information into apreviously-received image to generate an updated image. As additionaldifference information is received, the quality of the updated image isprogressively improved. Initially, the difference information representsa difference between the still image and the previously-received image.A memory responsive to the image constructor is configured and arrangedto store the updated image.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and advantages of the present invention will becomeapparent upon reading the following detailed description of variousembodiments and upon reference to the drawings in which:

FIG. 1 illustrates a videoconferencing system implementing an embodimentof the present invention;

FIG. 2A illustrates in block diagram form an embodiment of an encodingarrangement according to the present invention;

FIG. 2B is a flow chart illustrating the operation of the encodingarrangement illustrated in FIG. 2A;

FIG. 3A illustrates in block diagram form an embodiment of a decodingarrangement according to the present invention;

FIG. 3B is a flow chart illustrating the operation of the decodingarrangement depicted in FIG. 3A;

FIG. 4 illustrates in block diagram form another embodiment of anencoding arrangement according to the present invention;

FIG. 5 illustrates in block diagram form another embodiment of adecoding arrangement according to the present invention; and

FIGS. 6A-6B are a flow chart illustrating the operation of the encodingand decoding arrangement illustrated in FIGS. 4-5.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiment described. On the contrary, the invention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DESCRIPTION OF THE VARIOUS EMBODIMENTS

The present invention is believed to be applicable to a variety ofsystems and arrangements that transmit images. The present invention hasbeen found to be particularly advantageous in videoconferencingapplications in which still images are transmitted. While the presentinvention is not so limited, an appreciation of various aspects of theinvention is thus gained through a discussion of various applicationexamples operating in this type of application.

FIG. 1 illustrates an example videoconferencing system 10 particularlysuited for use in connection with the present invention. A firstvideoconferencing station 12 includes a first terminal 14 that receivesimages from a camera 16. The images are processed by a codec in theterminal 14 and are transmitted over a communications channel 20. Forexample, the images may be transmitted over a telephone line, an ISDNline, an Internet channel, or a similar communications network. Theimages are then received by a second videoconferencing station 22,including a second terminal configured to be compatible with the firstterminal. To facilitate the discussion below, the first and secondterminals 14 and 24 are respectively referred to as local and remoteterminals. It should be understood, however, that the features describedcan be incorporated into either or both terminals. The remotevideoconferencing station 22 may further include a camera 26 and a codec28. The remote videoconferencing station 22 receives the images over thecommunications channel 20 from the local videoconferencing station 12and decodes them using the codec 28. The decoded images may be displayedon a display monitor 30. In this manner, meetings can be conducted, forexample, with participants separated by long distances with all meetingparticipants able to see each other.

In order to transmit a still image quickly, the codec 18 initiallycompresses a still image as a relatively small data packet representingthe still image with relatively low quality. The data packet istransmitted over the communications channel 20 in a relatively shortperiod of time due to the small size of the data packet. The data packetis received by the remote videoconferencing station 22 and is decoded bythe codec 28, and the relatively low quality decoded image is displayedfor viewing.

Control of the still mode operation can be performed at one or bothstations. As described more fully below, in accordance with oneembodiment of the present invention only the transmitting (e.g., local)terminal needs to be equipped with still mode functionality. The localterminal uses standards-compliant image transmission techniques suchthat any remote terminal which complies with the standards for videotransfers will be capable of handling the still mode video informationtransmitted by the local terminal. Where both terminals include stillmode capability, either the local or remote station can initiate atransfer of a still mode image from the local terminal to the remoteterminal. For example, a user at the remote videoconferencing stationmay request via a control channel in the videoconferencing standardsthat a still mode image be sent. The requester can further select thedesired quality of the still image.

FIGS. 2A and 2B illustrate a, particular embodiment of an encodingarrangement 100 that can be used to transmit high quality still imagesin accordance with one aspect of the present invention. In theillustrated embodiment, a memory 101 stores an image received from avideo camera 102. In response to a still mode control signal from astill mode control block 103, transmission of images from the videocamera 102 to the memory 101 is suspended. Instead, the memory 101stores the last image received from the video camera 102 as a stillimage. The still image is sent to a difference information generator104, which is configured and arranged to calculate differenceinformation representing the difference between the still image and areconstructed image stored in a memory 106, such as a frame buffer.Initially, the memory 106 might not contain a reconstructed image, andthe difference information generator 104 passes the entire still imageto a variable quality encoder 108, which compresses the still image as afirst set of encoded data at step 122. This encoded data is stored in amemory 110, such as a buffer, for transmission using a communicationschannel 112 at a step 124.

Alternatively, the memory 106 may contain the last reconstructed image,which is also present at the remote terminal from a previoustransmission of an encoded image. If the memory 106 contains the lastreconstructed image, the difference information generator computesdifference information based on the still image stored in the memory 101and the reconstructed image stored in the memory 106, as depicted at ablock 123. As depicted at a block 125, the difference information isthen encoded as a first set of encoded data that represents thedifference image with a relatively low quality as determined by theinitial quality level used by the variable quality encoder 108. At ablock 127, the encoded data is stored in the memory 110 for transmissionusing the communications channel 112.

A reconstructor 114 constructs a reconstructed image based on the firstset of encoded data at step 126, as more fully described below, andstores the reconstructed image in the memory 106. Because the first setof data represents the still image at the relatively low quality, thereconstructed image has a lower quality than the still image. At step128, the encoding quality is increased.

The difference information generator 104 computes difference informationbased on the still image stored in the memory 101 and the reconstructedimage stored in the memory 106 at step 130. At step 132, the variablequality encoder 108 encodes the difference information at the increasedencoding quality according to an existing video compression standard fortransmitting difference information, such as the ITU-T H.261 or H.263recommendation. Standards of this type typically encode videoinformation as image data and a set of motion vector, which carryinformation relating to the motion of various subimages, such as pixelsor blocks, from one frame to the next. In one embodiment of theinvention, this type of standard may be used to encode the differenceinformation used in the still mode with the motion vector set to zero.In this manner, as described more fully below, a conventional decodercan be used to receive the still images since the transmitted data isfully compliant with these standards.

At step 134, the difference information is stored in the memory 110 fortransmission over the communications channel 112. As long as the stillmode is still enabled, new reconstructed images having successivelyincreasing qualities are constructed by iteratively incorporating newdifference information into the reconstructed image stored in the memory106. With each iteration, the new reconstructed image is stored in thememory 106, and new difference information is then computed as adifference between the still image stored in the memory 101 and the newreconstructed image. As described more fully below, the differenceinformation is also used by the remote terminal in conjunction with theoriginally transmitted low quality image to build iteratively a higherquality still image at the remote location. This process stops when thestill mode is disabled or when a desired quality is reached. Rather thanmodifying the quality of the difference information directly, theoriginal image could be repeatedly encoded at successively increasingqualities, with difference information between the original and encodedimages being calculated and processed for transmission over thecommunications channel 112 in each iteration.

FIGS. 3A and 3B illustrate a particular embodiment of a decodingarrangement 200 that can be used to receive high quality still modeimages in accordance with another aspect of the present invention. Adata stream is received over a communications channel 202 at step 222and is stored in a memory 204, such as a buffer. A decoder 206 decodesthe data stream into image data at step 224. The image data is receivedby a reconstructor 208, which constructs a first reconstructed image atstep 226. The first reconstructed image is stored in a memory 210, suchas a frame buffer, and is displayed on a display device 212 at step 228.The first reconstructed image may be displayed on, for example, acomputer monitor or a television. Additional data received over thecommunications channel 202 supplements the image data with differenceinformation representing the difference between the image stored in thememory 210 and a newly reconstructed higher quality representation ofthe original still image at the local terminal. The additional data isdecoded by the decoder 206 at step 230 and integrated into thereconstructed image by the reconstructor 208 at step 232. Thisintegrated image is then stored in the memory 210 as a new reconstructedimage for subsequent use. As the reconstructed image is developedprogressively, it is displayed by the display device 212 at step 234.

Additional difference information is received and decoded to repeatedlyincrease the quality of the reconstructed image until the desiredquality is achieved or until the still mode is disabled, therebyreturning the receiving terminal to real-time image transmission. Inthis manner, the receiving terminal receives a coarse image thatimproves in quality over time.

The decoding arrangement 200 can be implemented using a codec thatcomplies with an existing video compression standard, such as the ITU-TH.261 or H.263 recommendation. A typical H.261- or H.263-compliant codecincludes a demultiplexer 214, which parses the data stream received overthe communications channel 202 into image data and motion data. Theimage data is decoded by the decoder 206 and reconstructed by thereconstructor 208 as described above. The motion data is decoded by asecond decoder 216 into a set of motion vector. A motion processor 218modifies the image stored in the memory 210 according to the motionvector.

When a still mode enabler 220 is engaged, it transmits a request to atransmitting terminal at the other end of the communications channel202. In response to the request, the transmitting terminal progressivelytransmits a still image in the manner more fully described above withthe motion vector set to zero. With no motion information conveyed bythe motion vector, the second decoder 216, and the motion processor 218effectively perform no function in the decoding arrangement 200.

FIG. 4 illustrates another embodiment of an encoding arrangement 300that is particularly suited for use in the present invention. Anoriginal image is stored in a memory 301, such as a frame buffer. Theoriginal image is received by a summing element 302, which is configuredand arranged to calculate difference information representing thedifference between the original image and an image stored in a memory304, such as a frame buffer. When no image is initially stored in thememory 304, the original image is passed to a transformation block 306,which transforms the image into coefficient data selected from acontinuous range of values. In the illustrated example, thetransformation block 306 performs a discrete cosine transform (DCT) onthe original image.

The coefficient data is further transformed by a transformation block,illustrated as a quantizer 308, which is controlled by a coding controlblock 310. The quantizer 308 maps the coefficient data to a discrete setof values by dividing the continuous range of values into a set ofnon-overlapping subranges. Each subrange is mapped to a single value,such that whenever a coefficient falls within a given subrange, thequantizer 308 generates the corresponding discrete value. The size ofthe subranges and the quality of the quantization are controlled by thecoding control block 310.

The quantized data thus generated is encoded by an encoding block,illustrated as a Huffman encoder 312. For example, the quantized datamay be encoded as a run length vector. The encoded data is stored in amemory 314 for transmission over a communications channel 316.

The quantized data is also received by a transformation block 318. Thetransformation block 318, implemented as an inverse quantizer in theillustrated example, maps the quantized data to a continuous range ofvalues. The quality of the inverse quantization is controlled by thecoding control block 310. A second transformation block 320 furthertransforms the quantized data using an inverse discrete cosine transform(IDCT) to reconstruct the image as it would be seen at a remotevideoconferencing station. The reconstructed image is incorporated intothe image previously stored in the memory 304 by a summing element 322,and the image thus produced is stored in the memory 304. The memory 304provides the reconstructed image to the summing element 302, and theprocess is repeated, each time increasing the quality specified by thecoding control block 310 until a desired quality is achieved. In thismanner, the image is progressively built from a coarse representationhaving a relatively low quality to a sharp representation having ahigher quality.

FIG. 5 illustrates another embodiment of a decoding arrangement 400 thatis particularly suited for use in the present invention. A data streamreceived over a communications channel 402 is stored in a memory 404,implemented as a buffer in the illustrated example. A decoder,illustrated as a Huffman decoder 406, decodes the data stream into imagedata and provides the image data to a transformation block 408. Thetransformation block 408, implemented as an inverse quantizer in theillustrated example, maps the image data to a continuous range ofvalues. A second transformation block 410 further transforms thecontinuous value using an inverse discrete cosine transform (IDCT) toreconstruct the difference information received from the local terminal.A summing element 414 integrates the difference information into theimage previously stored in a memory 412, depicted as a frame buffer, andthe image thus produced is stored in the memory 412 and displayed by adisplay device. Additional data received over the communications channel402 is integrated into the stored image to improve its quality. As aresult, the image quality progressively improves as long as the stillmode is enabled.

The decoding arrangement 400 can be implemented using a codec thatcomplies with an existing video compression standard, such as the ITU-TH.261 or H.263 recommendation. A typical H.261- or H.263-compliant codecincludes a demultiplexer 416, which parses the data stream received overthe communications channel 402 into image data and motion data. Theimage data is decoded by the Huffman decoder 406 and transformed by thetransformation blocks 408 and 410 as described above. The motion data,on the other hand, is decoded by a second Huffman decoder 418 into a setof motion vector. A motion processor 420 modifies the image stored inthe memory 412 according to the motion vector and sends the modifiedimage to the summing element 414. With no motion information beingconveyed by the motion vector, the demultiplexer 416, the second Huffmandecoder 418, and the motion processor 420 effectively perform nofunction in the decoding arrangement 400. Thus, when a conventionaldecoder arrangement having this construction receives the still imageinformation transmitted from the local station, it progressivelyimproves the quality without any additional hardware or software.

The decoder arrangement may also include a still mode enabler 422, whichwhen engaged, transmits a signal to the local (transmitting) terminal atthe other end of the communications channel 402 to initiate a still modehigh quality image transmission. In response to the signal, thetransmitting terminal transmits a still image as described more fullyabove with the motion vector set to zero.

FIGS. 6A-6B illustrate a particular embodiment of a method 500 that canbe used to transmit and receive high quality still mode images inconnection with the encoding and decoding arrangements illustrated inFIGS. 4-5. If the still mode is enabled, an image is divided into blocksof pixels at step 502. For example, each block may be an eightpixel-by-eight pixel square.

Next, the coding control block 310 initializes a quantization parameterQP at step 504. The quantization parameter QP is used to control thequality of the block generated by the encoding arrangement 300 of FIG.4. For example, the quantization parameter QP may vary over a range of 1to 31, with quality increasing as the quantization parameter QPdecreases.

The transformation block 306 transforms each block according to adiscrete cosine transform (DCT) at step 508 to obtain a coefficientvector comprising a coefficient for each pixel of the block. Assumingthe image is divided into 8×8 blocks of pixels, for example, thecoefficient vector comprises sixty-four coefficients. The coefficientsmay vary over a continuous range of values. At step 510, the quantizer308 quantizes each coefficient to one of a discrete set of values bydividing the continuous range of values into a set of non-overlappingsubranges. Each subrange is mapped to a single value, such that whenevera coefficient falls within a given subrange, the quantizer 308 generatesthe corresponding discrete value at step 510. The size of the subrangesis determined by the quantization parameter QP. Larger values of QPproduce larger subranges and coarser quantization. Conversely, smallervalues of QP produce smaller subranges and finer quantization.Accordingly, coding control block 310 initializes the quantizationparameter QP to a relatively large value in step 504. For example, QPmay be initially set to a value of 31. Some applications, however,benefit from a lower initial value of the quantization parameter QP. Forexample, an image may have already been transmitted and received with aquality corresponding to a lower value of the quantization parameter QP.

The quantized data thus generated at step 510 is encoded at step 512into a set of encoded data. For example, the quantized data may beHuffman encoded by the Huffman encoder 312 as a run length vector. Theencoding arrangement 300 transmits the set of encoded data at step 514over the communications channel.

The set of encoded data is also inverse quantized by the transformationblock 318 at step 516 and transformed according to an inverse discretecosine transform (IDCT) by the second transformation block 320 at step518 to reconstruct a block as seen by a viewer at a remotevideoconferencing station. Because this reconstruction is based onquantized data, the reconstructed block has a lower quality than theoriginal block.

After all the blocks have been reconstructed through blocks 508 through518, the coding control block 310 determines whether the quantizationparameter QP is greater than one at a decision step 524. If thequantization parameter QP is greater than one and the still mode isstill enabled, the coding control block 310 decrements the quantizationparameter QP at step 528, thereby increasing the encoding quality.

At steps 532 through 544, the encoding arrangement 300 computes andencodes difference information for each of the blocks making up theoriginal still image. The summing block 302 computes differenceinformation as a difference between the original i^(th) block stored inthe memory 301 and the reconstructed i^(th) block generated at steps 516and 518 and stored in the memory 304.

Next, the transformation block 306 transforms the difference informationaccording to a DCT at step 534 to obtain a coefficient vector. Thequantizer 308 quantizes the difference information according to thedecremented quantization parameter QP at step 536. Because the codingcontrol block 310 decremented the quantization parameter QP at step 528,the difference information is quantized at a higher quality than at step510. The quantizer 308 thus generates level information at step 536.

The level information is encoded as a set of encoded information at step538. In the illustrated example, the Huffman encoder 312 encodes thelevel information as a run length vector. The encoding arrangement 300transmits the encoded information over the communications channel atstep 540.

The transformation block 318 inverse quantizes the level information atstep 542. The inverse quantized difference information is transformedaccording to an IDCT by the transformation block 320 to generate adifference block at step 544. At step 546, the reconstructed blockgenerated by the transformation block 320 at step 518 is updated byadding the difference block generated by the transformation block 320 atstep 544. It will be appreciated by those skilled in the art that steps542, 544, and 546 are also respectively performed by the transformationblocks 408, 410, and 414 of the decoding arrangement 400 at thereceiving terminal after the incoming data stream has been decoded bythe Huffman decoder 406.

After all of the blocks have been processed according to blocks 532through 544, the quantization parameter QP is successively decrementedand the blocks are reprocessed using the decremented quantizationparameter QP, thereby increasing the encoding quality at each iterationof steps 532 through 544. This process continues until either thequantization parameter QP is decremented to one or until the still modeis disabled by a user at the remote videoconferencing station.

The various embodiments described above are provided by way ofillustration only and should not be construed to limit the invention.Those skilled in the art will readily recognize various modificationsand changes which may be made to the present invention without strictlyfollowing the exemplary embodiments and applications illustrated anddescribed herein, and without departing from the true spirit and scopeof the present invention which is set forth in the following claims.

What is claimed is:
 1. For use in transmitting a still image at a desired quality using a communications channel, a system comprising:a still mode control block configured and arranged to selectively enable a still mode; a first memory configured and arranged to, when the still mode is not enabled, store a series of images in succession, and, when the still mode is enabled, store one of the series of images as the still image to be transmitted; a difference information generator configured and arranged to generate difference information representing a difference between a reconstructed image and the still image; a variable quality encoder configured and arranged to encode the difference information as encoded data representing the difference information at a variable quality and to successively increase the quality until a quality threshold is satisfied; an image reconstructor configured and arranged to construct the reconstructed image as a function of the difference information, the reconstructed image initially comprising a previously-transmitted image; and a second memory configured and arranged to store the reconstructed image.
 2. A system, according to claim 1, further comprising an image receiving device coupled to the first memory and configured and arranged to receive an image.
 3. A system, according to claim 1, further comprising an output memory, coupled to the communications channel and responsive to the variable quality encoder and configured and arranged to transmit the encoded data over the communications channel.
 4. A system, according to claim 1, wherein the variable quality encoder comprises a transformation block configured and arranged to transform the data into a form for quantization.
 5. A system, according, to claim 4, wherein the the variable quality encoder is configured and arranged to quantize the data based on the parameter, wherein a high parameter value indicates lower signal quality, and wherein transformation is via use of a discrete cosine transformation.
 6. A system, according to claim 1, wherein the image reconstructor incorporates the difference information into the first image.
 7. For use in transmitting a still image at a desired quality between first and second stations using a communications channel, a system comprising:a still mode control block configured and arranged to selectively enable a still mode; a first memory configured and arranged to, when the still mode is not enabled, store a series of images in succession, and, when the still mode is enabled, store one of the series of images as the still image to be transmitted; a difference information generator configured and arranged to generate difference information representing a difference between a function of a reconstructed image and the still image; a variable quality encoder configured and arranged to encode the difference information as encoded data representing the difference information at a variable quality and to successively increase the quality until a quality threshold is satisfied; an image reconstructor configured and arranged to construct a new reconstructed image as a function of the difference information and the previous reconstructed image; and a second memory configured and arranged to store the new reconstructed image.
 8. A system, according to claim 7, wherein the function of a reconstructed image is an identity function.
 9. A system, according to claim 7, wherein the still mode control block is configured and arranged to selectively enable a still mode function in response to an input from a remote one of the first and second stations.
 10. A system, according to claim 7, wherein the still mode control block is configured and arranged to selectively enable a still mode function in response to an input from a local one of the first and second stations.
 11. A system, according to claim 7, wherein a first one of the reconstructed image uses a previously transmitted image.
 12. A system, according to claim 7, wherein the still mode control block is configured and arranged to selectively enable a still mode function in response to an input from one of the first and second stations, and the desired quality is selected from said one of the first and second stations.
 13. For use in transmitting a still image at a desired quality between first and second stations using a communications channel, a system comprising:still mode control means for selectively enable a still mode; first memory means for, when the still mode is not enabled, storing a series of images in succession, and, when the still mode is enabled, storing one of the series of images as the still image to be transmitted; difference information generation means for generating difference information representing a difference between a function of a reconstructed image and the still image; variable quality encoding means for encoding the difference information as encoded data representing the difference information at a variable quality and for successively increasing the quality until a quality threshold is satisfied; image reconstruction means for constructing a new reconstructed image as a function of the difference information and the previous reconstructed image; and second memory means for storing the new reconstructed image.
 14. A method of causing a still image to be transmitted at a desired quality between first and second stations using a communications channel, comprising:selectively enabling a still mode from one of the first and second stations; when the still mode is not enabled, storing a series of images in succession, and, when the still mode is enabled, storing one of the series of images as the still image to be transmitted; generating difference information representing a difference between a function of a reconstructed image and the still image; using a variable quality encoding process, encoding the difference information as encoded data representing the difference information at a variable quality and successively increasing the quality until a quality threshold is satisfied; constructing a new reconstructed image as a function of the difference information and the previous reconstructed image; and storing the new reconstructed image. 